fdisk

SYNOPSIS
fdisk [-uc] [-b sectorsize] [-C cyls] [-H heads] [-S sects] device
fdisk -l [-u] [device...]
fdisk -s partition...
fdisk -v
fdisk -h
DESCRIPTION
fdisk (in the first form of invocation) is a menu-driven program for
creation and manipulation of partition tables. It understands DOS-type
partition tables and BSD- or SUN-type disklabels.
fdisk does not understand GUID partition tables (GPTs) and it is not
designed for large partitions. In these cases, use the more advanced
GNU parted(8).
fdisk does not use DOS-compatible mode and cylinders as display units
by default. The old deprecated DOS behavior can be enabled with the
'-c=dos -u=cylinders' command-line options.
Hard disks can be divided into one or more logical disks called parti-
tions. This division is recorded in the partition table, found in sec-
tor 0 of the disk. (In the BSD world one talks about `disk slices' and
a `disklabel'.)
Linux needs at least one partition, namely for its root file system.
It can use swap files and/or swap partitions, but the latter are more
efficient. So, usually one will want a second Linux partition dedi-
cated as swap partition. On Intel-compatible hardware, the BIOS that
boots the system can often only access the first 1024 cylinders of the
disk. For this reason people with large disks often create a third
partition, just a few MB large, typically mounted on /boot, to store
the kernel image and a few auxiliary files needed at boot time, so as
to make sure that this stuff is accessible to the BIOS. There may be
reasons of security, ease of administration and backup, or testing, to
use more than the minimum number of partitions.
DEVICES
The device is usually /dev/sda, /dev/sdb or so. A device name refers
to the entire disk. Old systems without libata (a library used inside
the Linux kernel to support ATA host controllers and devices) make a
difference between IDE and SCSI disks. In such cases the device name
will be /dev/hd* (IDE) or /dev/sd* (SCSI).
The partition is a device name followed by a partition number. For
example, /dev/sda1 is the first partition on the first hard disk in the
system. See also Linux kernel documentation (the Documenta-
tion/devices.txt file).
five cylinders. The remaining space in the volume header may be used
by header directory entries. No partitions may overlap with the volume
header. Also do not change its type or make some filesystem on it,
since you will lose the partition table. Use this type of label only
when working with Linux on IRIX/SGI machines or IRIX/SGI disks under
Linux.
A DOS-type partition table can describe an unlimited number of parti-
tions. In sector 0 there is room for the description of 4 partitions
(called `primary'). One of these may be an extended partition; this is
a box holding logical partitions, with descriptors found in a linked
list of sectors, each preceding the corresponding logical partitions.
The four primary partitions, present or not, get numbers 1-4. Logical
partitions start numbering from 5.
In a DOS-type partition table the starting offset and the size of each
partition is stored in two ways: as an absolute number of sectors
(given in 32 bits), and as a Cylinders/Heads/Sectors triple (given in
10+8+6 bits). The former is OK -- with 512-byte sectors this will work
up to 2 TB. The latter has two problems. First, these C/H/S fields
can be filled only when the number of heads and the number of sectors
per track are known. And second, even if we know what these numbers
should be, the 24 bits that are available do not suffice. DOS uses
C/H/S only, Windows uses both, Linux never uses C/H/S.
If possible, fdisk will obtain the disk geometry automatically. This
is not necessarily the physical disk geometry (indeed, modern disks do
not really have anything like a physical geometry, certainly not some-
thing that can be described in simplistic Cylinders/Heads/Sectors
form), but it is the disk geometry that MS-DOS uses for the partition
table.
Usually all goes well by default, and there are no problems if Linux is
the only system on the disk. However, if the disk has to be shared
with other operating systems, it is often a good idea to let an fdisk
from another operating system make at least one partition. When Linux
boots it looks at the partition table, and tries to deduce what (fake)
geometry is required for good cooperation with other systems.
Whenever a partition table is printed out, a consistency check is per-
formed on the partition table entries. This check verifies that the
physical and logical start and end points are identical, and that each
partition starts and ends on a cylinder boundary (except for the first
partition).
Some versions of MS-DOS create a first partition which does not begin
on a cylinder boundary, but on sector 2 of the first cylinder. Parti-
tions beginning in cylinder 1 cannot begin on a cylinder boundary, but
this is unlikely to cause difficulty unless you have OS/2 on your
machine.
A sync() and an ioctl(BLKRRPART) (reread partition table from disk) are
performed before exiting when the partition table has been updated.
Long ago it used to be necessary to reboot after the use of fdisk. I
bug in DOS FORMAT and DOS FDISK.
The bottom line is that if you use cfdisk or fdisk to change the size
of a DOS partition table entry, then you must also use dd to zero the
first 512 bytes of that partition before using DOS FORMAT to format the
partition. For example, if you were using cfdisk to make a DOS parti-
tion table entry for /dev/sda1, then (after exiting fdisk or cfdisk and
rebooting Linux so that the partition table information is valid) you
would use the command "dd if=/dev/zero of=/dev/sda1 bs=512 count=1" to
zero the first 512 bytes of the partition.
BE EXTREMELY CAREFUL if you use the dd command, since a small typo can
make all of the data on your disk useless.
For best results, you should always use an OS-specific partition table
program. For example, you should make DOS partitions with the DOS
FDISK program and Linux partitions with the Linux fdisk or Linux cfdisk
program.
OPTIONS
-b sectorsize
Specify the sector size of the disk. Valid values are 512,
1024, 2048 or 4096. (Recent kernels know the sector size. Use
this only on old kernels or to override the kernel's ideas.)
Since util-linux-2.17, fdisk differentiates between logical and
physical sector size. This option changes both sector sizes to
sectorsize.
-c[=mode]
Specify the compatiblity mode, 'dos' or 'nondos'. The default
is non-DOS mode. For backward compatibility, it is possible to
use the option without the <mode> argument -- then the default
is used. Note that the optional <mode> argument cannot be sepa-
rated from the -c option by a space, the correct form is for
example '-c=dos'.
-C cyls
Specify the number of cylinders of the disk. I have no idea why
anybody would want to do so.
-H heads
Specify the number of heads of the disk. (Not the physical num-
ber, of course, but the number used for partition tables.) Rea-
sonable values are 255 and 16.
-S sects
Specify the number of sectors per track of the disk. (Not the
physical number, of course, but the number used for partition
tables.) A reasonable value is 63.
-h Print help and then exit.
-l List the partition tables for the specified devices and then
by a space, the correct form is for example '-u=cylinders'.
-v Print version number of fdisk program and exit.
BUGS
There are several *fdisk programs around. Each has its problems and
strengths. Try them in the order cfdisk, fdisk, sfdisk. (Indeed,
cfdisk is a beautiful program that has strict requirements on the par-
tition tables it accepts, and produces high quality partition tables.
Use it if you can. fdisk is a buggy program that does fuzzy things -
usually it happens to produce reasonable results. Its single advantage
is that it has some support for BSD disk labels and other non-DOS par-
tition tables. Avoid it if you can. sfdisk is for hackers only -- the
user interface is terrible, but it is more correct than fdisk and more
powerful than both fdisk and cfdisk. Moreover, it can be used nonin-
teractively.)
These days there also is parted. The cfdisk interface is nicer, but
parted does much more: it not only resizes partitions, but also the
filesystems that live in them.
The IRIX/SGI-type disklabel is currently not supported by the kernel.
Moreover, IRIX/SGI header directories are not fully supported yet.
The option `dump partition table to file' is missing.
SEE ALSO
cfdisk(8), sfdisk(8), mkfs(8), parted(8), partprobe(8), kpartx(8)
AVAILABILITY
The fdisk command is part of the util-linux package and is available
from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.
util-linux June 2010 FDISK(8)